A total of 29 colorectal polyps were classified into those with mild, moderate, and severe dysplasia, and carcinoma. Morphometric analyses were performed on each group, determining; mean nuclear area (N-area), nuclear cytoplasmic (N/C) ratio, nuclear crowding index (Cx) and mean cellular area (C-area). N-area and N/C ratio increased in accordance with the progress of the atypical changes. Cx showed a gradual increase up to moderate dysplasia and a decline from severe dysplasia to carcinoma. N-area in severe dysplasia and carcinoma was distinctively larger than that in mild and moderate dysplasia. As for C-area, the value from normal to moderate dysplasia remained about the same, in contrast to the much higher levels in severe dysplasia and carcinoma. These results clearly indicated that the rise in N/C ratio from normal to moderate dysplasia depends mainly on nuclear crowding, but that from severe dysplasia to carcinoma on nuclear enlargement.